In recent years, there has been known a display unit having a so-called pivot function that allows an image-display screen of the display unit to be freely changed between a landscape position and a portrait position.
Among display units provided with such a pivot function, there is known a display unit having a so-called liquid crystal display screen. The liquid crystal display screen displays an image while receiving backlight by being driven according to image data (for example, see Patent Document 1 below).
When liquid crystals are employed to display an image, it is preferable that portions using the liquid crystals be backlighted. Among display units employing liquid crystals in a screen, there is known a display unit that obtains light for backlighting from a cold-cathode fluorescent tube that laterally extends along the top surface of the liquid crystal display screen when the screen is in a portrait position. In such a display unit, backlight is emitted, via a light-guiding plate disposed on the back surface of the liquid-crystal display screen, from the cold-cathode fluorescent tube that laterally extends along the top surface of the screen.
FIG. 1 is a diagram showing a display unit having such a liquid crystal display screen.
A display unit 100 shown in FIG. 1 includes: a liquid crystal panel 100a having a built-in liquid crystal display screen; and a support 100b for supporting the liquid crystal panel 100a. Part (a) of FIG. 1 shows the liquid crystal panel 100a in a landscape position, while Part (b) of FIG. 2 shows the same in a portrait position.
FIG. 2 is a schematic block diagram of the liquid crystal panel shown in FIG. 1.
The liquid crystal panel 100a shown in FIG. 2 includes a liquid crystal display screen 101a, a cold-cathode fluorescent tube 102a, and an inverter circuit 103a for supplying an AC voltage to drive the cold-cathode fluorescent tube 102a. 
Part (a) of FIG. 2 shows a state in which light is obtained from the cold-cathode fluorescent tube 102a that laterally extends along the top surface of the liquid crystal panel 100a when the liquid crystal panel 100a is in the landscape position.
In the liquid crystal panel 100a shown in FIG. 2, the liquid crystal display screen 101a and the cold-cathode fluorescent tube 102a are integrated. Therefore, after the liquid crystal panel 100a is changed from the landscape position to the portrait position, the cold-cathode fluorescent tube 102a is in such a position that it vertically extends along a flank of the liquid crystal display screen 101a as shown in Part (b) of FIG. 2.    Patent Document 1: Japanese Patent Application Publication No. 2000-182793
By the way, when the cold-cathode fluorescent tube 102a of the liquid crystal panel 100a is used in the landscape position, as shown in Part (a) of FIG. 2, i.e., there is almost no temperature difference among parts of the fluorescent tube 102a and thus, it is possible to prevent occurrence of a concentration difference among parts of gas filling the inside of the tube. The concentration difference in gas is a factor that shortens the lifetime of the fluorescent tube. In contrast, when the cold-cathode fluorescent tube 102a is used in the portrait position as shown in Part (b) of FIG. 2, there arises such a problem that a temperature difference occurs between an upper side and a lower side of the fluorescent tube 102a due to heat convection inside the fluorescent tube 102a, which makes the distribution of the gas uneven. This uneven distribution of the gas causes a concentration difference, thereby shortening the lifetime of the tube.
The above problem is not limited to cases where a cold-cathode fluorescent tube is used as a light source. Similar problems are likely to arise when other type of light source such as a hot-cathode fluorescent tube or an LED is used, as long as the light source in use is such a type that its lifetime can be shortened by the temperature difference among parts of the light source.